1. Field of the Invention
The present invention relates to a light source device capable of switching the radiation angle of illuminating light; to a display device provided with this light source device and capable of switching the viewing angle; to a terminal device equipped with this display device; to a light source unit incorporating the light source device; and to a method for driving the light source device.
2. Description of the Related Art
Because of their thin profile, light weight, small size, low energy consumption, and other advantages, display devices that use liquid crystals are currently deployed and used on a wide scale in a range of devices that includes monitors, televisions (TV), and other large terminal devices; notebook-type personal computers, cash dispensers, vending machines, and other mid-sized terminal devices; and personal TVs, PDAs (Personal Digital Assistance: personal information terminal), mobile phones, mobile gaming devices, and other small terminal devices. These liquid crystal display devices can be generally classified, according to the type of light source used, as transmissive, reflective, or transflective (using transmitted light and reflected light jointly). Energy consumption can be reduced in the reflective type, since external light can be utilized for display, but since contrast and other aspects of display performance are inferior compared to the transmissive type, transmissive and transflective liquid crystal display devices currently represent the mainstream. In transmissive and transflective liquid crystal display devices, a light source device is installed on the back surface of a liquid crystal panel, and a display is created using the light emitted from the light source device. Specifically, in current mainstream liquid crystal display devices, a light source device that is separate from the liquid crystal panel is essential.
In the liquid crystal panel that is the primary component of a liquid crystal display device, information is displayed by using an electric field to control the orientation of liquid crystal molecules, and numerous modes have been proposed according to the combination of the type and initial orientation of the liquid crystal molecules, the direction of the electric field, and other characteristics. Among these modes, the modes most often used in conventional terminal devices include an STN (Super Twisted Nematic) mode using a simple matrix structure, and a TN (Twisted Nematic) mode using an active matrix structure. However, liquid crystal panels that use these modes have a narrow angle range in which gradation can be correctly distinguished, and grayscale inversion occurs outside the optimum viewing position.
This problem of grayscale inversion was relatively insignificant in mobile phones and other terminal devices whose display content consists mainly of phone numbers and other text. However, with recent technological developments, terminal devices have come to display not only text information, but also large amounts of graphical information, and a resultant problem is that visibility of images is severely reduced by grayscale inversion. Liquid crystal panels that use a mode having a wide viewing angle range in which contrast can be correctly distinguished without the occurrence of grayscale inversion are therefore gradually being installed in terminal devices. Liquid crystal panels having this type of mode are referred to generically as wide-angle-view liquid crystal panels, and horizontal field modes such as IPS (In-Plane Switching) systems, multi-domain vertical alignment modes, and the like are implemented therein. Since gradation can be correctly distinguished over a wide viewing angle range by using these wide-angle-view liquid crystal panels, even though a medium-sized terminal device is basically a personal tool, applications for sharing information with others that can be appreciated by several people simultaneously are gradually being developed and installed.
On the other hand, the nature of medium-sized terminal devices is such that they are used not only in closed rooms under tight security, but also in public places. It then becomes important to keep display of private information and confidential information from being viewed by a third party. Particularly in recent years, occasions where private information and confidential information are displayed have increased in conjunction with progress in terminal devices, and demand for techniques to prevent surreptitious viewing is increasing. There accordingly exists a need to develop a technique capable of preventing surreptitious viewing and to enable the display to be viewed only by the user. This can be achieved by narrowing the angle range in which the display is visible, i.e., by narrowing the viewing angle range.
As described above, a display having a wide viewing angle range to allow viewing by several people simultaneously, and a display having a narrow viewing angle range that can be viewed only by the user, are both individually desirable, and the ability to switch between these two types of display in a single terminal device is also desirable. Therefore, in order to satisfy such requirements, there has been proposed a display device in which the light source device essential to the liquid crystal display device is designed so that the viewing angle range can be changed.
FIG. 1 is a schematic sectional view showing a first conventional viewing-angle-controlled liquid crystal display device described in Japanese Laid-open Patent Application No. 5-72529. As shown in FIG. 1, the first viewing-angle-controlled liquid crystal display device 1001 is composed of a liquid crystal element 1170 that is capable of controlling scattering; and a liquid crystal element 1180 that is capable of controlling optical rotation and double refraction properties. The liquid crystal element 1170 capable of controlling scattering is composed of substrates 1110 and 1111 that are optically transparent in the visible region, transparent electrodes 1120 and 1121, a scattering liquid crystal 1130, a voltage supply source 1100, and a switch 1190. The liquid crystal element 1180 capable of controlling optical rotation and double refraction properties is composed of substrates 1111 and 1112 that are optically transparent in the visible region, transparent electrodes 1122 and 1123, polarizers 1140 and 1141, orientation films 1150 and 1151, a liquid crystal layer 1160 having optical rotation and double refraction properties, a voltage supply source 1101, and a switch 1191. Polymer dispersed liquid crystal is used as the scattering liquid crystal 1130, and TN liquid crystal is used as the liquid crystal 1180 that is capable of controlling optical rotation and double refraction properties. The polarizers 1140 and 1141 are arranged as a crossed Nicol.
In the first viewing-angle-controlled liquid crystal display device configured in this way as described in Japanese Laid-open Patent Application No. 5-72529, a voltage is applied between the transparent electrodes 1122 and 1123, whereby the optical rotation and double refraction properties of the liquid crystal layer 1160 are changed, and this change can be used to control the transmittance of light. In this type of display mode that utilizes optical rotation and double refraction properties, the optical rotation and double refraction properties that substantially affect the incident light will differ according to the direction of the viewing angle. A phenomenon therefore occurs in which the luminance and chroma are reduced or inverted, depending on the viewing angle.
Accordingly, a liquid crystal element 1170 that can control scattering is disposed above this type of viewing-angle-dependent liquid crystal element 1180 to reduce viewing angle dependency. Specifically, since the liquid crystal molecules are randomly oriented in the absence of an electric field applied to the liquid crystal 1130 of the liquid crystal element 1170 that can control scattering, substantially isotropic scattering occurs over the entire viewing angle range, and a display can be obtained that has minimal dependency on the viewing angle. When an electric field is applied to the liquid crystal 1130, on the other hand, the liquid crystal molecules orient themselves substantially parallel to the applied electric field. The light emitted from the liquid crystal element 1180 exits without being scattered by the liquid crystal molecules. While visual characteristics are not improved and the viewing angle characteristics resemble those of a conventional TN liquid crystal in this case, it is possible for an image to be correctly recognizable only by a single user situated to the front of the screen. Consequently, when it is desirable that an image be correctly recognizable only by a single user situated to the front of the screen, surreptitious viewing by others can be prevented by not applying electrical current to the liquid crystal 1130.
FIG. 2 is a schematic sectional view showing a second conventional viewing-angle-controlled liquid crystal display device described in Japanese Laid-open Patent Application No. 9-244018; and FIG. 3 is a schematic perspective view showing the illumination device used in this viewing-angle-controlled liquid crystal display device. As shown in FIG. 2, the second conventional viewing-angle-controlled liquid crystal display device 2101 is composed of a liquid crystal display element 2102, a scatter control element (scatter control means) 2103, and an illumination device (backlight) 2104. The scatter control element 2103 is disposed between the liquid crystal display element 2102 and the illumination device 2104. As shown in FIG. 3, the illumination device 2104 is provided with an opaque slitted sheet (translucent sheet) 2120 and an irradiating unit 2121 disposed on the substrate side of the scatter control element 2103. A fluorescent tube or other light source 2122 is provided to the irradiating unit 2121, and a light emission surface 2123 for emitting the light from the light source 2122 and guiding the light to the opaque slitted sheet 2120 is formed. A reflecting sheet 2124 for reflecting the light emitted from the light source 2122 is provided in the irradiating unit 2121 on the surface facing the light emission surface 2123. In the opaque slitted sheet 2120, a large number of linear opaque members extending in one direction are arranged parallel to each other on one surface of a translucent sheet. The extension direction of the opaque members coincides with the vertical direction of the display screen.
In the second conventional viewing-angle-controlled liquid crystal display device configured as described in Japanese Laid-open Patent Application No. 9-244018, the light emitted from the light source 2122 exits from the light emission surface 2123 of the irradiating unit 2121, and is radiated to the scatter control element 2103 via the opaque slitted sheet 2120. When the light emitted from the light emission surface 2123 passes through the opaque slitted sheet 2120, the opaque slitted sheet 2120 blocks light incident from directions that are significantly inclined with respect to the light-incident surface of the opaque slitted sheet 2120. There is obtained thereby transmitted light that is highly parallel to the direction perpendicular to the plane of the opaque slitted sheet 2120. The light emitted from the illumination device 2104 then is incident on the scatter control element 2103. The scatter control element 2103 controls the scattering properties of incident light according to whether or not voltage is applied. When the scatter control element 2103 is in the scattering state, the light emitted from the illumination device 2104 is scattered by the scatter control element 2103, whereas when the scatter control element 2103 is in the transparent state, the light from the illumination device 2104 is not scattered.
In this second conventional viewing-angle-controlled liquid crystal display device 2101, highly collimated light emitted from the illumination device 2104 is scattered by the scatter control element 2103 and made incident on the liquid crystal display element 2102 when the scatter control element 2103 is in the scattering state. As a result, light that has passed through the liquid crystal display element 2102 escapes in all viewing angle directions of the display unit, and it becomes possible to discern display content from positions other than a position directly in front of the display unit. In contrast, when the scatter control element 2103 is in the transparent state, the highly collimated light from the illumination device 2104 enters the liquid crystal display element 2102 while still maintaining a high degree of collimation, since it is not scattered by the scatter control element 2103. As a result, light is not transmitted to locations from which the display unit is viewed diagonally to the left or right in the horizontal direction, so the screen appears dark, making it impossible to ascertain the displayed content. In other words, only an observer who is directly facing the display unit can ascertain the displayed content.
As described above, in this second conventional viewing-angle-controlled liquid crystal display device 2101, since scattering of the light can be controlled by the scatter control element 2103, the viewing angle characteristics of displayed content can be controlled. Furthermore, since highly collimated light can be directed towards the liquid crystal display element 2102 by the illumination device 2104, with the scatter control element 2103 in the transparent state, it is possible to reliably obtain viewing angle characteristics so that only an observer directly facing the display unit can ascertain the displayed content. Consequently, it is possible to obtain a liquid crystal display device that is capable of being switched at random between a state in which display characteristics are uniformly maintained in all viewing angle directions with negligible dependence of display characteristics on viewing angle, and a state in which displayed content can be ascertained only from a position directly facing the display unit.
However, the conventional viewing-angle-controlled liquid crystal display devices described above have problems such as the following. In the conventional viewing-angle-controlled liquid crystal display devices described above, the color of images changes when the viewing angle is switched. Particularly when a switch is made from a narrow viewing angle display having a narrow viewing angle range to a wide viewing angle display having a wide viewing angle range, images take on a yellowish hue, which can be unpleasant for the user.